Image caption appear as tooltip in XWiki when using frame or thumb options

With those two options, MediaWiki generates a figure with the text below the image.

For example:

[[File:Xwiki-logo.png|alt=alt|frame|400x400px|caption]]

The reason is that there was no way to express an image caption in the XWiki Rendering API before, but i should be possible to fix that with the new figure API.